The syndrome of medullary carcinoma of the thyroid, pheochromocytoma and parathyroid hyperplasia (Multiple Endocrine Neoplasm Syndrome - Type II) is transmitted genetically as an autosomal dominant with a high degree of penetrance. The frequency of medullary carcinoma in children of an affected parent is estimated to be approximately 50%. Annual screening of all family members 3 years of age and older in a large New Haven kindred has been commenced using a modification of the pentagastrin stimulation test reported by Hennessey and co-workers (J. Clin. Endocrinol. Metabl. 39:487, 1974). Family members are studied on an annual basis using the ambulatory facilities of the Adult (General) Clinical Research Center and of the Children's Clinical Research Center. A pentagastrin infusion, 0.5 mcg/kg is performed with serum for calcitonin immunoassay obtained before and at 2,5,10 and 20 minutes following. Radioimmunoassay of calcitonin is being performed in the laboratories of Dr. Armen H. Tashjian, Harvard School of Dental Medicine. Screening for parathyroid disease with total and ionized calcium, alkaline phosphatase and immunoreactive parathormone are obtained simultaneous with pentagastrin infusion. Plasma for determination of dopamine-beta-hydrosylase and 24 hour urine collection for measurement of urinary catecholamines is being performed in the laboratory of Dr. Lewis Landsberg, Beth Isreal Hospital, Boston. Approximately 120 family members have been located within the general New Haven area and are available for testing. Approximately 1/3 of these are children under 16 years of age. As of 3/1/77, 58 family members were studied with the remainder to be studied over the course of one year. Thus far, no abnormalities have been found in previously undiagnosed and asymptomatic family members screened for pheochromocytoma and parathyroid adenomas. Since the study commenced in mid-October 1976, results of the calcitonin response to pentagastrin infusions were not available as of the preparation of this report.